Visual contents, be they still or moving images, are in general creations which benefit from copyright-related exclusivity guarantees. Their reproduction is in general permitted only within a strongly defined framework which allows for remuneration of authors and their beneficiaries.
In order to ensure that these legal rules are correctly adhered to, numerous systems have been develop to prevent illegal copies or sufficiently impair their quality as to render them unusable.
Within this framework, patent application EP 1 237 369 aims to combat the copying of images by picture-taking during their display, for example with a camcorder in a cinema auditorium. With this aim, it is proposed that the brightness of the pixels of a pattern be modulated around the value to be displayed at a frequency which renders the pattern invisible to the human eye but which generates artefacts on the sequence filmed by the camcorder. This pattern is commonly called anti-copy pattern. The shape of the pattern can be determined so as to inscribe messages of the type “ILLEGAL COPY” which will appear in the images displayed by the camcorder.
In order for the pattern to be invisible to the human eye, the modulation consists in alternating images in which the pattern is bright with images in which it is dark, the mean brightness of the pattern over several images corresponding to that to be displayed in the images in the absence of a pattern. During the display of these images, the eye carries out integration and in fact perceives the mean brightness. This technique may be also applied to the colour of the images by alternating images in which the pattern is more coloured with images in which it is less so, the mean colour of the pattern over several images corresponding to that to be displayed in the absence of the pattern.
In practice, each image of the initial sequence received at 24 Hz is decomposed into several images at N*24 Hz in which the brightness and/or the colour of some parts of the images are modulated to create the anti-copy pattern as indicated above.